Gas discharge tube

ABSTRACT

A gas discharge tube  10  of the present invention generates an electric discharge between an anode part  24  and a cathode part  52  disposed inside a sealed container  12  in which gas has been sealed. The gas discharge tube  10  includes an electric discharge path restricting part  28  which is cylindrical and disposed between the anode part and the cathode part, and which has a throughhole  46  for narrowing an electric discharge path between the anode part and the cathode part; and a supporting part  30  which supports the electric discharge path restricting part and which is electric insulating. The electric discharge path restricting part is provided at its tip portion with a projecting part  44  which is cylindrical and projects toward the cathode part side. Letting the outer diameter and height of the projecting part  44  be “D” and “H,” respectively, then D/H is in the range of 0.5 to 2.0. With this structure, it is possible to generate an intense electric field in the vicinity of the tip of the projecting part. This can decrease a startup voltage, thereby ensuring the generation of an electric discharge.

TECHNICAL FIELD

The present invention relates to a gas discharge tube such as a heavyhydrogen lamp to be used particularly as a light source forspectroscopy, chromatography, etc.

BACKGROUND ART

As conventional techniques in the above-described field, those disclosedin Japanese Unexamined Patent Publication No. H7-288106 and JapaneseUnexamined Patent Publication No. H10-64479 are known. In either gasdischarge tube, a barrier wall made of metal is disposed on an electricdischarge path between an anode part and a cathode part, and a smallhole is formed on the barrier wall so as to narrow the electricdischarge path. In such a structure, light with high brightness can beobtained by the small hole on the electric discharge path. Inparticular, in the gas discharge tube of Japanese Unexamined PatentPublication No. H7-288106, brightness is further increased by extendingthe length of the small hole, that is, a portion of the electricdischarge path that is narrowed. On the other hand, in the gas dischargetube disclosed in Japanese Unexamined Patent Publication No. H10-64479,higher brightness is achieved by disposing a plurality of barrier wallsin addition to extending the length of the hole to be used.

The demand for higher brightness in the technical field of gas dischargetubes has been comparatively satisfied by the techniques disclosed inthe above-described patent publications.

However, when a portion of the electric discharge path that is narrowedis extended in length, an electric discharge is less liable to occur. Toavoid this problem, in the gas discharge tube disclosed in JapaneseUnexamined Patent Publication No. H10-64479, a plurality of metalbarrier walls are disposed to generate an electric discharge step bystep; however, this results in complicating a power supply circuit.

Therefore, an object of the present invention is to provide a gasdischarge tube which can securely generate an electric discharge,regardless of the length of a portion of the electric discharge paththat is narrowed.

DISCLOSURE OF THE INVENTION

In order to achieve the aforementioned object, the present inventionprovides a gas discharge tube comprising: a sealed container in whichgas is contained; an anode part disposed in the sealed container; acathode part defining an electric discharge part for generating anelectric discharge with the anode part, the cathode part being disposedinside the sealed container in such a manner as to be distanced from theanode part; an electric discharge path restricting part beingcylindrical and conductive and having a throughhole for narrowing theelectric discharge path, the electric discharge path restricting partbeing disposed between the anode part and the cathode part, and beingelectrically connected with an external power source; and anelectric-discharge-path-restricting-part supporting part which supportsthe electric discharge path restricting part and which is electricallyinsulating, wherein the electric discharge path restricting part has aprojecting part which is cylindrical and projects toward the cathodepart side, and a ratio (D/H) of an outer diameter (D) of the projectingpart to a height (H) of the projecting part is in a range of 0.5 to 2.0.

In such a structure where the electric discharge path restricting partprojects toward the cathode part, and D/H is in the range of 0.5 to 2.0,the electric discharge path restricting part and the cathode part have anon-uniform electric field therebetween, and can generate an intenseelectric field in the vicinity of the tip of the projecting part,thereby decreasing a startup voltage. This facilitates the generation ofa startup discharge, thereby ensuring the generation of the maindischarge.

It is effective to make the outer diameter of the projecting part of theelectric discharge path restricting part be in the range of 1.0 mm to2.0 mm. This can effectively generate the startup discharge to begenerated between the cathode part and the electric discharge pathrestricting part exclusively at the tip portion and in the vicinity ofthe throughhole in the projecting part of the electric discharge pathrestricting part.

It is also preferable that the throughhole in the electric dischargepath restricting part includes a small hole part which is provided onthe anode part side and has a constant inner diameter, and an increaseddiameter hole part which is linked with the small hole part and extendstoward the cathode part side while increasing in diameter toward thecathode part side. The small hole part mainly functions as a part fornarrowing the electric discharge path, and the increased diameter holepart forms an excellent arc ball inside, thereby contributing to highbrightness. In addition, in a condition where the increased diameterhole part has a conical inner peripheral surface, a depth (A) in therange of 0.3 mm to 1.3 mm and an opening angle (θ) in the range of 60°to 90°, it becomes possible to form a further stable arc ball.

The aforementioned features and advantages and other features andadvantages of the present invention will be made clear to those skilledin the art through the following detailed description with reference toaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view showing a gas discharge tube according to a firstembodiment of the present invention.

FIG. 2 is an enlarged cross sectional view of an electric discharge pathrestricting part in the gas discharge tube shown in FIG. 1.

FIG. 3 is an end view showing a gas discharge tube according to a secondembodiment of the present invention.

FIG. 4 is an end view showing a gas discharge tube according to a thirdembodiment of the present invention.

FIG. 5 is an enlarged cross sectional view of an electric discharge pathrestricting part in the gas discharge tube shown in FIG. 4.

BEST MODES FOR CARRYING OUT THE INVENTION

Now, preferable embodiments of the gas discharge tube of the presentinvention will be described in detail with reference to accompanyingdrawings. In the following description, it is to be understood thatvarious terms indicating directions such as “upeardly”, “dowinwardly”and the like are referred to based on the conditions of correspondingdrawings for the sake of convenience, and should not be construed aslimiting terms.

FIRST EMBODIMENT

FIG. 1 shows an end view of a gas discharge tube according to a firstembodiment of the present invention that is cut in the directionorthogonal to the axis (tube axis). A gas discharge tube 10 shown inFIG. 1 is a side-on type heavy hydrogen lamp. Specifically, the gasdischarge tube 10 has a sealed container 12 made of glass in whichseveral hundreds of Pa of heavy hydrogen gas has been sealed. The sealedcontainer 12 comprises a side tube part 14 which is cylindrical andsealed at one end thereof, and a stem part (not shown) for sealing theother end of the side tube part 14. A portion of the side tube part 14is used as a light emitting window 18. The sealed container 12accommodates a light emission part assembly 20 therein.

The light emission part assembly 20 includes a base part 22 which iselectrically insulating, made of ceramics or the like. The base unit 22is disposed opposed to the light emitting window 18. Above the base part22 is formed a tabular anode part 24. Onto the rear side of the anodepart 24, a tip portion of a stem pin 26 is fixedly connectedelectrically. The tip portion extends in the direction of the tube axis(the center axis of the side tube part 14), and stands on the stem part.

The light emission part assembly 20 also has anelectric-discharge-path-restricting-part supporting part (hereinafterreferred to as supporting part) 30 for supporting an electric dischargepath restricting part 28 that will be described later. The supportingpart 30 is fixed on the upper surface of the base part 22. Thesupporting part 30 is thicker than the anode part 24, and has a concavepart 32 on the bottom surface center thereof to dispose the anode part24. When the anode part 24 is disposed in the concave part 32 and thesupporting part 30 is fixed to the base part 22, the anode part 24 issandwiched between the stem pin 26 and the supporting part 30. Thesupporting part 30 also has an opening 34 in its center, which formspart of the electric discharge path.

Furthermore, a conductive plate 36 is disposed on the upper surface ofthe supporting part 30. The conductive plate 36 is electricallyconnected with the tip portion (nor shown) of a stem pin 38 standing onthe stem part. The conductive plate 36 has an opening 40 in its center,which is disposed coaxially with the opening 34 of the supporting part30 so as to form part of the electric discharge path when the conductiveplate 36 is fixed on the supporting part 30.

The light emission part assembly 20 also includes a cathode part 52which is disposed outside the light path on the light emission window 18side. The cathode part 52 is provided for generating thermal electrons,and more specifically, is formed by coating electron emitting materialonto a coil which is extended in the tube axial direction and is made oftungsten. The cathode part 52 is electrically connected with the tipportion of an unillustrated stem pin standing on the stem part via aconnection pin so as to allow feeding of electric power from outside.

As clearly shown in FIG. 2, the electric discharge path restricting part28 is shaped like a cylinder, and is provided with a flange part 42 forfixing at the end on the conductive plate 36 side. The cylindrical partof the electric discharge path restricting part 28 that projects upwardfrom the flange part 42 is referred to as a projecting part 44, and theprojecting part 44 has an outer diameter “D” preferably in the range of1.0 mm to 2.0 mm. In addition, letting the height or amount ofprojection of the projecting part 44 be “H,” the relationship D/Hbetween “H” and the outer diameter “D” of the projecting part 44 ispreferably in the range of 0.5 to 2.0. The inside of the electricdischarge path restricting part 28 makes a throughhole 46 for narrowingthe electric discharge path, and the throughhole 46 is formed of a smallhole part 48 which is provided on the anode part 24 side in such amanner as to have a constant inner diameter, and of an increaseddiameter hole part 50 which is linked with the small hole part 48 andextends upward while increasing in diameter in a funnel shape. The smallhole part 48 is a part for mainly narrowing the electric discharge path,and has an inner diameter of approximately 0.5 mm. The increaseddiameter hole part 50 is a part for mainly forming an arc ball, and inthe illustrated embodiment, has a conical inner peripheral surface. Theincreased diameter hole part 50 has a depth (length) “A” preferably inthe range of 0.5 mm to 1.3 mm, and an opening angle θ preferably in therange of 60° to 90°.

The throughhole 42 of the electric discharge path restricting part 28 isformed of a small hole part 46 which is provided on the anode part 24side in such a manner as to have a constant inner diameter, and of anincreased diameter hole part 48 which is linked with the small hole part46 and extends upward while increasing in diameter in a funnel shape.The small hole part 46 is mainly for narrowing the electric dischargepath, and the increased diameter hole part 48 is mainly for forming anarc ball, and in the present embodiment, has a cone-shaped innerperipheral surface. In order to narrow the electric discharge path, itis preferable that the small hole part 46 has an inner diameter D1 of0.5 mm or so. In addition, it is preferable that a maximum innerdiameter D2 of the increased diameter hole part 48, that is, the innerdiameter D2 of the throughhole 42 on the end surface on the cathode sidebe in the range of 1 mm to 3 mm, and it is further preferable that D2/D1or the ratio of the inner diameter D2 to the diameter D1 of the smallhole part 46 is in the range of 4 to 10.

The light emission part assembly 20 also includes an electric dischargedistributor 54 made of metal and a front surface cover 56 in order toprevent materials spattered or evaporated from the cathode part 52 fromadhering to the light emission window 18. The electric dischargedistributor 54 is disposed to surround the cathode part 52 and is fixedon the upper surface of the supporting part 30. The front surface cover56 is opposite the electric discharge distributor 54 and is fixed on theupper surface of the supporting part 30. Between the electric dischargedistributor 54 and the front surface cover 56, a light passage opening62 for letting discharge light pass through is formed. The electricdischarge distributor 54 has an opening 60 formed in a portion thatfaces the front surface cover 56, and thermal electrons generated in thecathode part 52 pass through the opening 60.

Now, operations of the above-mentioned gas discharge tube 10 will bedescribed.

First, before an electric discharge, for 20 seconds or so, electricpower of approximately 10 W is supplied to the cathode part 52 from acathode external power source (nor shown) via a stem pin (nor shown) soas to preheat a coil composing the cathode part 52. Next, a voltage ofapproximately 160V is applied between the cathode part 52 and the anodepart 24 from a main discharge external power source (nor shown) via thestem pin 26, thereby preparing an arc discharge.

Later, a predetermined voltage is applied between the electric dischargepath restricting part 28 and the anode part 24 via the stem pins 38 and26 from a trigger external power source (nor shown). As a result, astartup discharge occurs between the cathode part 52 and the tip of theprojecting part 44 of the electric discharge path restricting part 28projecting on the cathode part 52 side.

In this embodiment, the electric discharge path restricting part 28 hasa shape that makes the ratio D/H between the outer diameter “D” of theprojecting part 44 and its height “H” be in the range of 0.5 to 2.0, sothat the electric discharge path restricting part 28 and the cathodepart 52 have a non-uniform electric field therebetween, and have anintense electric field particularly in the vicinity of the tip of theprojecting part 44. This can decrease a startup voltage for generatingthe startup discharge. Since the outer diameter “D” of the projectingpart 44 is in the range of 1.0 mm to 2.0 mm, the startup discharge to begenerated between the cathode part 52 and the electric discharge pathrestricting part 28 can be effectively generated exclusively in thevicinity of the increased diameter hole part 50 of the projecting part44 of the electric discharge path restricting part 28. This alsofacilitates the generation of the startup discharge.

The successful generation of the startup discharge ensures thegeneration of a main discharge (arc discharge) between the cathode part52 and the anode part 24 by the main discharge external electrode. Aftergenerating the main discharge, the electric power from the cathodeexternal power source is adjusted to optimize the temperature of thecathode part 52. This maintains the main discharge between the cathodepart 52 and the anode part 24, and forms an arc ball inside theincreased diameter hole part 50 of the projecting part 44 of theelectric discharge path restricting part 28. Since the electricdischarge path is narrowed with a sufficient length in the electricdischarge path restricting part 28 and the arc ball is formed,ultraviolet rays generated are released outside as light with extremelyhigh brightness after passing through the light emission window 18 ofthe sealed container 12 from the light passage opening 58 between theelectric discharge distributor 54 and the front surface cover 56. In thepresent embodiment the inner peripheral surface of the increaseddiameter hole part 50 of the projecting part 44 is conical; the depth“A” of the increased diameter hole part 50 of the projecting part 44 isin the range of 0.5 mm to 1.3 mm; and the opening angle θ of theincreased diameter hole part 50 is in the range of 60° to 90°, so thatthe arc ball is formed in a stable and excellent shape. Consequently,the light to be emitted is stable in brightness and amount.

SECOND EMBODIMENT

FIG. 3 is an end view showing a gas discharge tube according to a secondembodiment of the present invention that is cut along the axialdirection. A gas discharge tube 110 is a head-on type heavy hydrogenlamp, and has a sealed container 112 made of glass in which severalhundreds of Pa of heavy hydrogen gas has been sealed. The sealedcontainer 112 includes a side tube part 114 which is cylindrical; a stempart 116 for sealing the bottom end side of the side tube part 114; anda light emission window 118 for sealing the top end side of the sidetube part 114. The sealed container 112 accommodates a light emissionpart assembly 120.

The light emission part assembly 120 includes a base part 122 which isdiscoid and electrically insulating, made of ceramics or the like. Thebase unit 122 is disposed opposed to the light emitting window 118. Overthe base part 112 is formed an anode part 124. With the anode part 124,a tip portion of a stem pin 126 extending in the direction of the tubeaxis (the center axis of the side tube) standing on the stem part 116 isconnected electrically. The stem pin 126 is wrapped with an electricallyinsulating tube 127 made of ceramics or the like so as not to be exposedbetween the stem part 116 and the base part 122.

The light emission part assembly 120 also has anelectric-discharge-path-restricting-part supporting part (supportingpart) 130, which is electrically insulating, made of ceramics or thelike. The supporting part 130 is disposed and fixed onto the uppersurface of the base part 122. In the center of the supporting part 130,a circular opening 134 is formed, into which the main portion of theanode part 124 is accommodated. When the main portion of the anode part124 is disposed in the opening 134 and the supporting part 130 is laidand fixed onto the base part 122, the peripheral part of the anode part124 is sandwiched between the supporting part 130 and the base part 122.The opening 134 of the supporting part 130 forms a part of the electricdischarge path.

In addition, on the upper surface of the supporting part 130 is disposeda conductive plate 136. The conductive plate 136 is electricallyconnected with the tip portion of the stem pin 138 standing on the stempart 116. The stem pin 138 is also wrapped with an electricallyinsulating tube 139 made of ceramics or the like so as not to be exposedbetween the stem part 116 and the base part 122. The conductive plate136 is provided with a circular opening 140 smaller than the innerdiameter of the opening 134 of the supporting part 130. The opening 140is disposed to be coaxial with the opening 134 of the supporting part130 in a condition where the conductive plate 136 is fixed to thesupporting part 130, thereby forming part of the electric dischargepath.

In the center of the upper surface of the conductive plate 136, anelectric discharge path restricting part 128 made of metal for narrowingor restricting the electric discharge path from the anode part 124 isfixedly welded in such a manner as to be coaxial with the openings 134and 140. This enables electric power to be fed to the discharge pathrestricting part 128 from outside via the conductive plate 136 and thestem pin 138.

The electric discharge path restricting part 128 is substantiallyequivalent to the electric discharge path restricting part 28 of thefirst embodiment, that is, the one shown in FIG. 2. Therefore, when itis briefly described with the same reference marks and with reference toFIG. 2, the electric discharge path restricting part 128 is a cylinderhaving the flange 42 at one end thereof; the outer diameter “D” of theprojecting part 44 is preferably in the range of 1.0 mm to 2.0 mm;letting the height of the projecting part 44 be “H,” the relationshipD/H with the outer diameter “D” of the projecting part 44 is preferablyin the range of 0.5 to 2.0. The small hole part 48 of a throughhole 146in the electric discharge path restricting part 128 has an innerdiameter “d” of approximately 0.5 mm; the depth (length) “A” of theincreased diameter hole part 150 is preferably in the range of 0.5 mm to1.3 mm; and the opening angle θ is preferably in the range of 60° to90°.

The light emission part assembly 120 also includes a cathode part 152which is disposed outside the light path on the light emission window118 side. The cathode part 152 is provided for generating thermalelectrons, and to be more specific, is formed by coating electronemitting material onto a coil which is extended in the tube axialdirection and is made of tungsten. The cathode part 152 is electricallyconnected with the tip portion of an unillustrated stem pin standing onthe stem part 116 via a connection pin so as to allow feeding ofelectric power from outside.

The light emission part assembly 120 further includes an electricdischarge distributor 154 made of metal and a front surface cover 156 inorder to avoid materials spattered or evaporated from the cathode part152 from adhering to the light emission window 118. The electricdischarge distributor 154 is disposed to surround the cathode part 152and fixed on the upper surface of the supporting part 130. The frontsurface cover 156 is opposed to the electric discharge distributor 154and is fixed on the upper surface of the supporting part 130. Betweenthe electric discharge distributor 154 and the front surface cover 156,a light passage opening 158 for letting discharge light pass through isformed. The electric discharge distributor 154 has an opening 160 in aportion that faces the front surface cover 156, and thermal electronsgenerated in the cathode part 152 pass through the opening 160.

The gas discharge tube 110 according to the second embodiment thusstructured has the electric discharge path restricting part 128 which issubstantially the same as its equivalent in the gas discharge tube 10 ofthe first embodiment, although there is a difference between a head-ontype and a side-on type. In addition, the gas discharge tube 110 doesnot have a functional difference in the other parts thereof from the gasdischarge tube 10, thereby bringing about effects of requiring a lowvoltage for a startup discharge and securing the generation of thestartup discharge and the arc discharge. Furthermore, since the formedarc ball has a stable, excellent shape, the light to be emitted has highbrightness and is sufficient and stable in amount. A detaileddescription of the behavior of the gas discharge tube 110 will beomitted because it is equal to that of the gas discharge tube 10.

THIRD EMBODIMENT

FIG. 4 is an end view showing a gas discharge tube according to a thirdembodiment of the present invention that is cut in the directionorthogonal to the axis (tube axis). Similar to the gas discharge tube 10of the first embodiment, the gas discharge tube 210 of the thirdembodiment is a side-on type heavy hydrogen lamp. Specifically, the gasdischarge tube 210 has a sealed container 212 made of glass in whichseveral hundreds of Pa of heavy hydrogen gas is contained. The sealedcontainer 212 is formed of a side tube part 214 which is cylindrical andsealed at one end thereof, and a stem part (nor shown) for sealing theother end of the side tube part 214. A portion of the side tube part 214is used as a light emitting window 218. The sealed container 212accommodates a light emission part assembly 220.

The light emission part assembly 220 includes a base part 222 which iselectrically insulating, made of ceramics or the like. The base unit 222is disposed opposed to the light emitting window 218, and has a concavepart 223 on its upper surface. Over the base part 222 is formed atabular anode part 224, and onto the rear side of the anode part 224, atip portion of a stem pin 226, which extends in the direction of thetube axis and which stands on the stem part is fixedly connectedelectrically.

The light emission part assembly 220 also has anelectric-discharge-path-restricting-part supporting part (supportingpart) 230, which is tabular and made of ceramics or the like. Thesupporting part 230 is fixed on the top end surface of the outerperipheral surface of the base part 222. The supporting part 230 has aconcave part 232 on the bottom surface center thereof. The bottomsurface (downward surface) and side surfaces of the concave part 232 aredistanced from the anode part 224 by a predetermined spacing. Thesupporting part 230 also has an opening 234 in its center.

In the opening 234 of the supporting part 230 is provided an electricdischarge path restricting part 228 which is cylindrical and made ofmetal for narrowing the electric discharge path from the anode part 224.The attachment can be carried out by engaging the electric dischargepath restricting part 228 into the opening 234 and welding them;however, in the third embodiment as clearly shown in FIG. 5, a femalescrew 235 is formed on the opening 234, and a male screw 237 is formedon the outer surface of the end of the electric discharge pathrestricting part 228, thereby screwing them.

As shown in FIG. 5, part of the electric discharge path restricting part228 is screwed into the opening 234 of the supporting part 230, so thatthe part 244 corresponding to the projecting part 44 of the electricdischarge path restricting part 28 shown in FIG. 2 projects from theupper surface of the supporting part 230. The outer diameter “D” of theprojecting part 244 is preferably in the range of 1.0 mm to 2.0 mm,similar to the electric discharge path restricting part 28 shown in FIG.2. Letting the height “H” of the projecting part 244 be “H,” therelationship D/H with the outer diameter “D” of the projecting part 244is preferably in the range of 0.5 to 2.0. The electric discharge pathrestricting part 228 also has a throughhole 246 for narrowing theelectric discharge path, and the throughhole 246 is formed of a smallhole part 248 having a constant inner diameter, and of an increaseddiameter hole part 250 which increases in diameter upward like a cone.The small hole part 248 is a part for mainly narrowing the electricdischarge path, and has an inner diameter “d” of approximately 0.5 mm.The increased diameter hole part 250 is a part for mainly forming an arcball, and has a depth “A” preferably in the range of 0.5 mm to 1.3 mmand an opening angle θ preferably in the range of 60° to 90°.

Furthermore, a conductive plate 236 is provided along the bottom andside surfaces of the concave part 232 of the supporting part 230. Theconductive plate 236 is electrically connected with the tip portion of astem pin 238 standing on the stem part. The conductive plate 236 isprovided with an opening 240 aligned with the opening 234 of thesupporting part 230. The portions partitioning the opening 240 of theconductive plate 236 are electrically connected with the bottom end ofthe electric discharge path restricting part 228. This enables electricpower to be fed to the discharge path restricting part 228 from outsidevia the conductive plate 236 and the stem pin 238.

The light emission part assembly 220 also includes a cathode part 252which is disposed outside the light path on the light emission window218 side. The cathode part 252 is electrically connected with the tipportion of an unillustrated stem pin standing on the stem part via aconnection pin so as to allow feeding of electric power from outside.

The light emission part assembly 220 further includes an electricdischarge distributor 254 which is made of metal and surrounds thecathode part 252, and a front surface cover 256 which is formed inparallel with the electric discharge distributor 254 in order to avoidmaterials spattered or evaporated from the cathode part 252 fromadhering to the light emission window 218. These are fixed on the uppersurface of the supporting part 230, and between them is formed a lightpassage opening 258 for letting discharge light pass through. Theelectric discharge distributor 254 has an opening 260 through which topass thermal electrons generated in the cathode part 252.

The gas discharge tube 210 of the third embodiment is lit as follows. Inthe same manner as in the gas discharge tube 10 of the first embodiment,for 20 seconds or so before an electric discharge, electric power ofapproximately 10W is supplied to the cathode part 252 from a cathodeexternal power source (nor shown) via a stem pin (nor shown) so as topreheat the cathode part 252, and then a voltage of approximately 160Vis applied between the cathode part 252 and the anode part 224 from amain discharge external power source (nor shown) via the stem pin 226,thereby preparing an arc discharge. Later, a predetermined voltage isapplied between the electric discharge path restricting part 228 and theanode part 224 via the stem pins 238 and 226 from a trigger externalpower source (nor shown). As a result, a startup discharge occursbetween the cathode part 252 and the tip of the projecting part 244 ofthe electric discharge path restricting part 228 that projects towardthe cathode part 252 side. The occurrence of the startup discharge isfollowed by the generation of a main discharge between the cathode part252 and the anode part 224 due to a main discharge external electrode.Later, the electric power from the cathode external power source isadjusted to optimize the temperature of the cathode part 252. Thismaintains the main discharge between the cathode part 252 and the anodepart 224, and forms an arc ball inside the increased diameter hole part250 of the projecting part 244 in the electric discharge pathrestricting part 228.

The outer diameter “D” of the projecting part 244; the ratio of theouter diameter “D” to the height “H,” that is, D/H; and the depth “A”and opening angle θ of the increased diameter hole part 250 with aconical shape in the electric discharge path restricting part 228 of thethird embodiment are equal to those of the aforementioned electricdischarge path restricting parts 28 and 128. This makes it possible todecrease the startup voltage for generating the startup discharge, andalso to generate an arc ball with a stable, excellent shape, therebystabilizing the brightness and amount of light to be emitted.

As described hereinbefore, the gas discharge tube of the presentinvention can obtain high brightness because of the provision of theelectric discharge path restricting part for narrowing an electricdischarge path. The gas discharge tube can also decrease the startupvoltage because of the unique shape of the electric discharge pathrestricting part, regardless of its length, thereby facilitating thegeneration of the startup discharge. The easy generation of the startupdischarge secures the generation of the main discharge. Furthermore, theshape of the electric discharge path restricting part stabilizes thebrightness and amount of the light emitted.

Although the present invention and its advantages can be understoodhereinbefore, it is obvious that the above-described embodiments areonly typical preferable embodiments, and various modifications can becarried out in shape, structure and arrangement, without deviating fromthe spirit and scope of the present invention or losing the substantialadvantages.

1. A gas discharge tube comprising: a sealed container in which gas iscontained; an anode part disposed in said sealed container; a cathodepart defining an electric discharge part for generating an electricdischarge with said anode part, said cathode part being disposed insidesaid sealed container in such a manner as to be distanced from saidanode part; an electric discharge path restricting part beingcylindrical and conductive and having a throughhole for narrowing saidelectric discharge path, said electric discharge path restricting partbeing disposed between said anode part and said cathode part, and beingadapted to be electrically connected with an external power source; andan electric-discharge-path-restricting-part supporting part forsupporting said electric discharge path restricting part, saidsupporting part being electrically insulating, wherein said electricdischarge path restricting part has a projecting part which iscylindrical and projects toward said cathode part side, and a ratio(D/H) of an outer diameter (D) of said projecting part to a height (H)of said projecting part is in a range of 0.5 to 2.0.
 2. The gasdischarge tube according to claim 1, wherein the outer diameter of saidprojecting part of said electric discharge path restricting part is in arange of 1.0 mm to 2.0 mm.
 3. The gas discharge tube according to claim1, wherein said throughhole in said electric discharge path restrictingpart includes a small hole part which is provided on said anode partside and has a constant inner diameter, and an increased diameter holepart which is linked with said small hole part and extends toward saidcathode part side.
 4. The gas discharge tube according to claim 3,wherein said increased diameter hole part has a conical inner peripheralsurface, a depth (A) in a range of 0.3 mm to 1.3 mm and an opening angle(θ) in a range of 60° to 90°.
 5. The gas discharge tube according toclaim 1, wherein the gas in said sealed container is heavy hydrogen gas.